Elevated temperature dependence of the anisotropic visible-to-ultraviolet dielectric function of monoclinic beta-Ga2O3

We report on the temperature dependence of the dielectric tensor elements of $n$-type conductive $\beta$-Ga${}_2$O${}_3$ from 22${}^{\circ }$C-500${}^{\circ }$C in the spectral range of 1.5~eV–6.4~eV. We present the temperature dependence of the excitonic and band-to-band transition energies and their eigenpolarization vector orientations. We utilize a Bose-Einstein analysis of the temperature dependence of the observed transition energies and reveal electron coupling with average phonon temperature in excellent agreement with the average over all longitudinal phonon plasmon coupled modes reported previously [M. Schubert~$\mathit{\text{et}}\mathit{\text{al.}}$, Phys. Rev. B $\mathbf{\text{93}}$, 125209 (2016)]. We also report a linear temperature dependence of the wavelength independent Cauchy expansion coefficient for the anisotropic below-band-gap monoclinic indices of refraction.